Water flooding process



Unit

The present invention concerns an improved waterflooding process, andmore particularly, inhibiting ferric hydroxide plugging of water-floodinjection wells.

It is known that the presence of oxygen in water-flooding media promotescorrosion of metal equipment contacting such media and the conversion ofsoluble ferrous compounds into insoluble ferric hydroxide. Due to itsgelatinous nature, ferric hydroxide will quickly plug a water-floodinjection well. As a consequence, it is common practice to use closedinjection systems to keep contact of the injection fluids with theatmosphere at a minimum.

Even in a closed system, however, some oxygen is likely to leak into theinjection fluid. Moreover, aqueous media from which the injection fluidsare prepared often contain as much as parts per million by weight of dissolved oxygen. Such amounts of oxygen in the presence of soluble ferrousions under pH conditions normally employed in water flooding operations,i.e., from neutral to slightly basic, cause the formation of ferrichydroxide.

Techniques that have been employed to minimize this problem include theuse of ferric ion chelating agents. For instance, it is known tominimize well plugging by incorporating citric acid in the flood waters.This converts the insoluble ferric ions to soluble chelates which areswept on into the formation being flooded by the driving injectionfluids. While this technique is highly effective for the intendedpurpose, it is most disadvantageous when practiced in conjunction withthe addition of polymeric, mobility control agents to the injectionfluids. The presence of citric acid in the injection fluid upsets thenormal equilibrium between insoluble ferric species and their solubleferrous precursors. As a result, the conversion of ferrous to ferricspecies proceeds at an accelerated rate, even in the presence of verysmall amounts of oxygen. It has been discovered that this effect causessevere degradation of any polymeric additives that may be present in theinjection fluids. Such degradation leads to decreased etficiency interms of decreased mobility control of the injection fluids.

Another technique, which would appear to have potential for the controlof insoluble ferric species, is the use of a chemical, oxygen scavengerin the injection water. In fact, it has been suggested to employ certainchemicals of this class, such as soluble sulfite ions or hydrazine, asoxygen scavengers to minimize corrosion of equipment utilized forcarrying out the water flooding process. While the sulfite or hydrazinewill prevent corrosion and related introduction of ferrous ions into theinjection fluids, very small amounts of oxygen will oxidize ferrous ionsalready in the fluid to ferric hydroxide, even when corrosion iseffectively controlled by an oxygen scavenger.

It would be desirable, and it is an object of the present invention, toprovide a novel water flooding process in which formation plugging withferric hydroxide is effectively controlled. More especially, it is anobject to obviate ferric hydroxide plugging with reagents which maintainthe effectiveness of polymeric co-additives incorporated into theinjection fluids for mobility control. A still further object is toprovide non-corrosive injection fluids in which the oxidation of ferrousto ferric ions is effectively inhibited. A further benefit flowing fromthe discovery of the present invention, is the provision of a treatmentfor aqueous injection fluids in which any in d States Patent 0 "icesoluble ferric hydroxide already present therein is r: duced to solubleferrous ions. The above objects, an other benefits as will be apparenthereinafter, are aCcoII plished in accordance with the presentinvention.

The present invention concerns an improved water flooc ing process forthe secondary recovery of oil in whic aqueous fluid is injected into anoil bearing formation an displaced oil is recovered from the sameformation. Pa] ticularly, the invention concerns incorporating into thaqueous injection fluid, a small but effective amount 0 a water-solublehydrosulfite, such as for example potas sium, sodium or ammoniumhydrosulfite, to obviate 0 at least reduce, the plugging of the oilformation by ferri hydroxide. Incorporation of the hydrosulfiteeliminate oxygen-promoted corrosion of equipment and, at the sam time,prevents the oxidation of ferrous chemical specie to insoluble ferrichydroxide. A most valuable aspect o the use of the hydrosulfite ion isits compatability witl water soluble, polymeric mobility control agentsthat ma: be incorporated into the injection fluid. Such use of thhydrosulfite ion has a further and most unique advantagr of reducing andsolubilizing insoluble ferric hydroxide which may be present in theinjection fluid prior to in corporation of the hydrosulfite additive.

In carrying out the invention, the hydrosulfite additivr is incorporatedinto an aqueous injection fluid in any con venient manner. The amountused is preferably just thr minimum required to prevent plugging.Beneficial result: are realized, however, using as little as 5 up to asmuch a: 10,000 parts per million by weight of the additive, basec on theweight of the injection fluid. Although, it is no necessary, it ispreferred to practice the invention in 2 closed injection system, i.e.,a system in which contact with the atmosphere is kept to a minimum. Whenpolymeric mobility control agents are also being incorporatec into theaqueous injection fluid, the polymer and hydrosulfite may be addedsimultaneously or in any order. It is best, however, to incorporate thehydrosulfite additive prior to the addition of the polymeric additive.This minimizes the amount of mixing required subsequent to the additionof the polymer, and moreover, ideally conditions the aqueous injectionfluid for receiving the. polymeric additive by eliminating one of theconditions i.e., the presence of oxygen, which can promote moleculardegradation of the polymer.

Having prepared an aqueous injection fluid containing a hydrosulfiteadditive of the invention, with or without a water-soluble polymericmobility control agent, the aqueous injection fluid is introducedthrough an injection well, or wells, into the oi1-bearing formation. Thedisplaced oil is recovered at one or more producing wells removed fromthe injection well, or it may be recovered through a second passage inthe injection wells according to the single well secondary recoveryprocess of US. Patent 3,172,470. Aqueous liquids that may be employed toprepare the injection fluids include fresh Water, oil field brines,ocean water and in general any aqueous fluid which may be available atthe oil field site for practice of the invention.

Water-soluble, polymeric mobility control agents often employed includethe acrylic amide polymers such as homopolymers of acrylamide andcopolymers thereof with acrylic acid, methacrylic acid and alkali-metalsalts of such acids. Other classes of polymeric mobility control agentsinclude the water-soluble ,alkylene oxide polymers, polymer sulfonates,polyvinyl alcohols, esters and amides of styrene-maleic anhydridecopolymers, and in general any of a wide number of water-soluble, highmolecular weight polymers which enhance the viscosity of aqueous fluids.Teachings with respect to such polymers are found in US Patents2,731,414; 2,827,964; 2,842,492; 3,018,826; 3,039,529; 3,079,337; and3,085,063.

he following examples provide further illustration of present invention.

Example I ie. In the polymer, about 20 percent of the amide 1 ips hadbeen hydrolyzed to sodium carboxylate 1P5. Enough of the polymer wasincorporated into brine to provide 0.50 percent by weight polymer. The.e-polymer solution was divided into 350 milliliter uots which wereindividually treated as shown in the )wing table. After thirty minutes,the viscosity of the ted brine polymer solution was determined with a nviscosimeter.

TABLE I quot Treatment Viscosity 12 ml. of H 12 %p(.)p.m. of sodiumhydrosulfite in 12 ml. of 11. 2

100 p.p.m. of sodium hydrosulfite plus 10 11.2

p.p.m. of Fe++ in 12 ml. of H 0.

100 p.p.m. of sodium citrate and 100 ppm. of 3. 7

Fe++ in 12 ml. of H 0.

ll'iis refers to parts per million by weight based on the weight of thee.

be above data illustrate that the hydrosulfite has comitively littleadverse effect on the viscosity of the brine zmer solution and that thehydrosulfite is much suor in the presence of soluble ferrous ions, tosodium ate, which chelates ferric ion as it is formed. the polymer brinesolution is permitted to remain in tact with air, it will eventuallyabsorb enough oxygen :lestroy the effectiveness of the hydrosulfite as areing agent. As this occurs, polymer degradation may worse than that ofthe brine-polymer solution without additive. For instance, after twentyhours at room perature and in the presence of air, aliquot number ad ameasured viscosity of 12.2 centipoises in cont to that of aliquot 2which had dropped to 9.7 tipoises. Although this is not nearly ,assevere, as de- :lation caused by the use of the chelating agent forcontrol of ferric ions, it is a serious loss of viscosity, thus, abilityto control mobility of the brine in the bearing formation. For thisreason it is preferred when vmeric mobility control additives areemployed con- :tively with the hydrosulfite, that the combination beized in a closed injection system, i.e., one in which tact with theatmosphere is minimized or completely ided.

Example II This example illustrates the benefit to be realized inpractice of the invention as it may be applied to the paration ofinjection fluids from aqueous fluids coniing suspended ferric hydroxide.

F0 96 milliliters of an aqueous 3 percent sodium chlosolution was addedone milliliter of a 2 percent by .ght water solution of sodiumbicarbonate. Then, 1 liliter of an essentially oxygen-free solution of0.5 cent by weight ferrous sulfate hydrate (FeSO -7H O) 5 added. Thisprovided the solution with about ten ts per million by weight of ferrousions. The solution 5 stirred gently in the presence of .air untilfloccules of ric hydroxide were visible in the solution. At this pointthe solution had taken on a definite yellow color. Dissolved oxygen wasstripped from the solution by bubbling nitrogen through it. The ferrichydroxide floccules and yellow color remained in the solution afterremoval of the oxygen.

To 98 milliliters of the test solution prepared above was added twomilliliters of a 2% by weight water solution of sodium hydrosulfite (NaS O After about twenty minutes, the solution became clear and colorless.The ferric hydroxide floccules had completely disappeared.

To a like aliquot of the ferric hydroxide suspension was added onemilliliter of the sodium hydrosulfite solution. The result wassubstantially identical to that obtained above.

As a comparative experiment, two milliliters of a four percent by weightsodium sulfite (Na SO solution was added to a third comparable aliquotof the ferric hydroxide suspension. After sixteen hours, the ferrichydroxide had settled with no apparent diminution in quantity. At thispoint, the addition of two milliliters of the above sodium hydrosulfitecaused the precipitate and color to disappear in about 30 minutes.

In still further tests like that above, but using sodium hypophosphite(Na I-I PO H O) and sodium thiosulfate (Na S O in place of the sodiumsulfite, it was again shown that the sodium hydrosulfite was unique inits ability to solubilize pre-existing ferric hydroxide.

Example III In a field operation, ferric hydroxide plugging of injectionWells was encountered upon the start-up of a water flooding operation inwhich a polymer was used in the flooding medium to improve :mobilitycontrol. The polymer used was a partially hydrolyzed polyacrylamide inwhich about 25% of the initially available carboxamide groups had beenconverted sodium carboxylate groups. About by weight polymer was addedto the flooding medium. The resulting flooding medium was characterizedby a resistance property (R) equal to about 10.

The term resistance property as used herein is defined by D. Pye inImproved Secondary Recovery by Control of Water Mobility, Society ofPetroleum Engineers, AMIE, Paper No. SPE845.

As a result of the oxidation of soluble ferrous species to ferrichydroxide, polymer degradation occurred to the extent there was a 55%loss in the resistance property of the solution prior to injection.Subsequently, air leaks into the closed system were minimized. Thisreduced the resistance property loss to about 35%. 50 parts per millionby weight of sodium hydrosulfite was then added to the flooding medium.This reduced the resistance property loss to practically zero.

What is claimed is:

1. In a process for the secondary recovery of oil which comprisesinjecting an aqueous fluid containing a dispersed iron compound into anoil-bearing formation, the improvement which comprises incorporatinginto the injection fluid a water-soluble, hydrosulfite in an amountsufficient to prevent ferric hydroxide plugging of the formation.

2. A method as in claim 1 wherein the hydrosulfite is added to theinjection fluid as an alkali metal salt.

3. In a process for the secondary recovery of oil which comprisesinjecting into an oil bearing subterranean formation an aqueous solutionof a water-soluble, mobilitycontrolling polymeric additive and adispersed iron compound, the improvement which comprises incorporatinginto the injection fluid a water-soluble, hydrosulfite in an amountsuflicient to prevent ferric hydroxide plugging of the formation.

4. A method as in claim 3 wherein the hydrosulfite is added to theinjection fluid as an alkali metal salt.

5. A process for the secondary recovery of oil which comprises injectinginto an oil bearing subterranean formation an aqueous solution havingincidental contact with an iron-containing metal during the injectionprocess, said solution containing a water-soluble, mobility-controllingpolymeric additive and a Water soluble hydrosulfite, the lattercomponent being incorporated into the injection fluid in an amountsufficient to prevent ferric hydroxide plugging of the formation;whereby oil is displaced in the oil-bearing formation.

6. A process as in claim 5 wherein the polymeric additive is awater-soluble acrylamide polymer and the hydrosulfite is added to theinjection fluid as an alkali metal salt.

6 References Cited UNITED STATES PATENTS 2,787,326 4/1957 Hughes 166--423,163,213 12/1964 Bernard 166-S 3,203,480 8/!1965 Froning 166-93,258,070 6/1966 Reusser l669 3,258,072 6/4966 Froning 166-9 CHARLES E.OCONNELL, Primary Examiner. 0 JAMES A. LEPPINK, Examiner.

1. IN A PROCESS FOR THE SECONDARY RECOVERY OF OIL WHICH COMPRISESINJECTING AN AQUEOUS FLUID CONTAINING A DISPERSED IRON COMPOUND INTO ANOIL-BEARING FORMATION, THE IMPROVEMENT WHICH COMPRISES INCORPORATINGINTO THE INJECTION FLUID A WATER-SOLUBLE, HYDROSULFITE IN AN AMOUNTSUFFICIENT TO PREVENT FERRIC HYDROXIDE PLUGGING OF THE FORMATION.